The dual-core Athlon64 X2's are some of the best-performing consumer-line processors currently on the market. Great for heavy multi-tasking since the two cores can essentially perform multiple tasks simultaneously without a glitch.
If you're going for a new machine, I don't see any reason why you should go for a 32-bit CPU at this stage. If you're concerned about compatibility (and rightfully so), you can run the regular 32-bit version of XP, along with all your 32-bit drivers and applications, on a x86-64 CPU without a problem. You'll still benefit from the performance of the processor, although you won't really be using its 64-bit capabilities, per se.
Moreover, by opting for a 64-bit CPU, you're making your new system relatively future-proof (if there's even such a thing in the tech world!). If and when 64-bit systems and software become mainstream in the near future, you'll have a machine that's ready for the transition.
Hope that helps.
